Magnetic resonance imaging (MRI) involves the transmission of radio frequency (RF) energy. RF energy may be transmitted by a coil. Resulting magnetic resonance (MR) signals may also be received by a coil. In early MRI, RF energy may have been transmitted from a single coil and resulting MR signals received by a single coil. Later, multiple receivers may have been used in parallel acquisition techniques. Using multiple receivers facilitates speeding up signal reception, which in turn may reduce scan time. Similarly, multiple transmitters may be used in parallel transmission techniques. Using multiple transmitters may facilitate speeding up a transmission process, which in turn may facilitate volumetric excitation, selective isolation, and other high speed features.
However, conventional parallel transmission techniques have encountered issues with scaling, fidelity, and synchronization. Additionally, conventional transmission techniques that attempt to excite multiple frequencies have experienced setup and tuning issues associated with interactions between multiple coils used to excite multiple frequencies. For example, tuning a first coil to excite a first frequency may cause a second, previously tuned coil to behave differently than the coil would otherwise. Thus, experiments employing multi-frequency excitation may have been performed infrequently.
Conventional systems may have attempted to parallelize their existing RF transmitters. Thus, conventional systems may have relied on multiple, individually powered, single channel, analog-in-analog-out RF transmitters for parallel transmission. These systems tended not to scale well due to cabling duplication, power transmitter duplication, control duplication, and other issues. Even when a small number (e.g., 4) of transmitters were employed, these systems may not have produced a desired fidelity. For example, conventional systems may have had complicated power distribution management and may have been difficult to synchronize. Additionally, conventional systems typically had poor isolation between coils, resulting in degraded performance. Furthermore, these systems may have required each element in an array to be tuned and matched. Tuning and matching each element is a very time-consuming procedure.
Conventional systems may also have been limited by their use of relatively low power (e.g., <50 W), low efficiency class A or class AB amplifiers. While some systems may have included on-coil series and/or shunt-fed class-D amplifiers, even these conventional systems have suffered from several limitations including inadequate detuning and low efficiency. Due, at least in part, to these limitations, conventional systems may not have produced desired levels of amplitude and/or phase control and thus may have had less than desirable fidelity. Once again this may have limited the frequency with which multi-frequency experimentation may have been attempted.